Protective system



A ril 21, 1925. 1,534,697

A. S. FITZGERALL'D.

PROTECTIVE SYSTEM Filedlug. 13, 1924 k .r f n r@ w o t t t mt A V Ins Hm Patented Apr. 21, 1925.

UNITED STATES 1,534,692 PATENT OFFICE.

ALAN STEWART FITZGERALD, OF LONDON, ENGLAND, ASSIGNOR TO GENERAL ELEC-TR-IC COMPANY, A CORPORATION OF NEW YORK.

Application filed August 13, 1924.

To all whom it away concern:

it known that I, ALAN STEWART Frrzunnsrn, a subject of the BritishEmpire, residing at London, England, have invented certain new anduseful Improvements in ifi ing improved operating characteristics.

Current transformers for use in connection with high tension circuits,especially the kind known as bushing transformers, are inefiicient incomparison with low tension cur- 2o rent transformers, owing to thegreater space occupied by the primary insulation of the former. Thecurrent'output from a high tension current transformer, moreover, issmall in comparison with that from a low tension current transformerowing to the fewer primary ampere turns of the former (assuming a singleprimary turn in each). Owing to these facts, difficulty has beenexperienced in accurately balancing, over a wide range of primary load,the secondary currents of current transformers in circulating currentprotective systems for protect ing circuits of different potentials orthe current transformers of which have different characteristics. Anobject of my invention is to provide means for overcoming thisdifficulty. It is, therefore, especially applicable to systems andapparatus arranged for the protection of a transformer, or of a pair ofparallel feeders at different potentials.

According to my invention, the combined effect of the series connectedsecondary windings of two current transformers upon an additionalsecondary winding of one of the current transformers is neutralized bythe primary winding thereof, when the primary currents are of normalvalues, that is, when there is no fault in. the circuit that is beingprotected. The additional secondary wind ing is connected to a trippingdevice or protective relay and serves to operate the same in the eventof the normal relationship between the currents of the primaryconductors Jeing disturbed, such as may be caused by a fault.

PROTECTIVE SYSTEM.

Serial No. 731,867.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims. I

In the accompanying drawing, Fig. 1. represents a simple arrangement ofcirculating current protection according to prior practice, and Fig. 2an arrangement embodying my invention.

Like reference numerals are used when ever possible to denote similarparts and in 1 the cores 10, 11 surround the high tension primary 12 andthe lower tension primary 13 respectively. The secondary windings.latand 15 respectively, are connected for circulating current, aprotective relay 16 being arranged across equi-potential points tooperate in a manner well understood.

In Fig. 2 two secondary windings 1%, 14' are provided upon the core ofthe current transformer which has the poorer characteristic, that isupon the core 10 of the current transformer associated with the higherpotential primary 12. There is usually am ple space on a bushingtransformer for this purpose. The secondary winding 14 of thedouble-wound current transformer is con nected with the secondarywinding 15 of the other current transformer in such sense and is soarranged that current induced in the latter winding and flowing throughthe former will produce in the core 10 of the double-wound currenttransformer a magnetic flux equal and opposite to that produced thereinby the primary winding 12 thereof, when conditions in the circuit thatis being protected are normal. In these circumstances the resultant fluxin the core 10 of the double-wound current transformer is substantiallyzero since the magnetomotive force in the core 10 due to the seriesconnected secondary windings 14c and 15 is neutralized by themagnetomotive force of the primary winding 1:2. The other or additionalsecondary winding 14 is connected to a tripping device or protectiverelay 16 and will be energized only when the normal relation in theprotected circuit is disturbed. lVhen it is so energized, it may operateto open the main protected circuit, to give a warning signal, or in anyother desired manner. Owing to the relatively large current output fromthe current transformer associated with the circuit of the lowerpotential, here is no difficulty in maintaining the desired ratio ofampere turns between the series connected secondary windings l4 and 15.

In some cases it may be necessary to introduce a restraining bias in amanner that is well known, The impedance representing the restrainingwinding may the-n be excited from a winding on the core of the currenttransformer which does not carry the additional winding, that is, on thecore 11 of the current transformer associated with primary 13.Preferably it is connected in the circuit of the sericsconnected windings 14, 15.

If'at a given instant the currents in the primary windings 12 and 13 areflowing in the directions indicated by the arrows 17, then thedirections of currents in thesecondary winoings 14; and 15 are indicatedby the arrows 18 in normal conditions. A con sideration of the windings12 and let in Fig.

2 will show that they produce fluxes in the core 10 in opposite senses.Consequently, under normal conditions, that is, the desired currentbalance in the conductors 12 and 13, substantially no current flows inthe auXil-. iary secondary winding 14 and the relay 16 is not operated.Upon an unbalance in the currents in the conductors 12 and13, however,the resultant fiux in the core 10 will not be zero and current will flowin the secondary winding 14:. This current, depending upon the degree ofunbalance for which the relay 16 is set to operate, will actuate therelay.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

l. A protective arrangement for an alternating electric current circuitwherein two current transformers have their primary windings connectedin said circuit and their secondary windings connected in series,characterized by the fact that one of said transformers is provided withan additional secondary winding arranged so that the combined effectthereon of said series connected secondary windings is neutralized bythe primary winding of said one transformer when circuit conditions arenormal.

2. A. protective arrangement for an alternating electric current circuitcomprising two current transformers haying primary windings connected insaid circuit and secondary windings connected in. series, an auxiliarysecondary winding on one of said transformers and a relay connected incircuit with said auxiliary winding, the connection and arrangement ofsaid secondary windings being such that under normal conditions on saidcircuit the combined effect of said series connected secondary windingsis neutralized by the primary winding of said one transformer] V 3. Aprotective arrangement for a pair of conductors carrying alternatingelectric currents at different potentials comprising a magnetic corearound each conductor, a winding on each of said cores, said windingsbeing connected and arranged to produce under normal conditions on saidconductors a magnetomotiye force in the core around the conductorrat thehigher potential neutralized by the magnetomoti e force due to thecurrent in the higher potential conductor whereby the resultant flux inthe core is normally substantially zero, an auxiliary winding on thecore of the higher potential conductor, and a'relay connected in circuitwith said auxiliary winding.

In witness whereof, I have hereunto set my hand this thirtieth day ofJuly 192%.

ALAN STEWART FITZGERALDv

